Current orthopedic immobilization or support materials, e.g., casting tapes, are composed of a fabric backing and a curable compound such as plaster-of-paris or a synthetic resinous material. The fabric used in the backing serves several important functions. For example, it provides a convenient means of delivering the curable compound. It also helps reinforce the final composite cast. Furthermore, for an orthopedic casting material that incorporates a curable resin, use of a backing material with numerous voids, i.e., a backing with an apertured configuration, ensures adequate porosity. This allows a sufficient amount of curing agent, such as water, to contact the resin and initiate cure. This also ensures that the finished cast is porous, breathable, and comfortable for the patient.
The fabric used in many of the backings of orthopedic casting materials on the market is made of fiberglass. Such fiberglass backing materials generally provide casts with strength superior to casts that use synthetic organic fiber knits, gauze, nonwovens, and other nonfiberglass composite backings. Although fiberglass backing materials provide superior strength, they are of some concern to the medical practitioner during the removal of casts. Because casts are removed using conventional oscillatory cast saws, fiberglass dust is typically generated. Although the dust is generally classified as nonrespirable nuisance dust, and therefore not typically hazardous, many practitioners are concerned about the effect inhalation of such fiberglass dust particles may have on their health. Furthermore, although casts containing fiberglass generally have improved x-ray transparency compared to that of plaster-of-paris casts, the knit structure is visible, which can interfere with the ability to see fine detail in a fracture.
In developing backing materials for orthopedic casts, conformability of the material is an important consideration. In order to provide a "glove-like" fit, the backing material should conform to the shape of the patient's limb receiving the cast. This can be especially difficult in areas of bony prominences such as the ankle, elbow, heel, and knee areas. The conformability of a material is determined in large part by the longitudinal extensibility, i.e., lengthwise stretch, of the fabric.
Conformable fiberglass backings have been developed, however, special knitting techniques and processing equipment are required. To avoid the need for special techniques and equipment, nonfiberglass backing materials have been developed to replace fiberglass. However, many of the commercially available nonfiberglass backings, such as those containing polyester or polypropylene, also have limited extensibility, and thus limited conformability. Furthermore, the casts made from low modulus organic fibers are significantly weaker than casts made from a fiberglass casting tape. That is, the modulus of elasticity (ratio of the change in stress to the change in strain which occurs when a fiber is mechanically loaded) for many nonfiberglass materials (about 5-100 g per denier), e.g., polyester (about 50-80 grams per denier), is far lower than that for fiberglass (about 200-300 grams per denier) and as such provides a lower modulus, less rigid, cured composite. For this reason, the resin component of the cured composite needs to support a far greater load than it does when fiberglass fabric forms the backing. Thus, greater amounts of resin are generally required with nonfiberglass backings. This is not desirable because large amounts of curable casting compound may result in resin pooling, high exotherm, and reduced cast porosity.
The extensibility, and thereby conformability, of some fiberglass or polyester knit backing materials has been improved by incorporating elastic yarns into the wales of a chain stitch. The use of a backing that incorporates highly elastic yarns is not necessarily desirable, however, because of the possibility of causing constriction and further injury to the limb if the casting tape is not carefully applied. The constriction results from a relatively high elastic rebound force. Thus, inelastic or only slightly elastic stretch is preferred. A second characteristic that can be a drawback of these backings is the tendency to wrinkle longitudinally when the backing is extended. This results in decreased conformability and a rough surface.
Thus, a need exists for a backing material that is sufficiently conformable to a patient's limb, has low potential for constriction, resists wrinkling during application, and provides a cured cast that exhibits high strength, rigidity, and porosity. Also, a need exists for a backing material that is radiolucent, e.g., transparent to x-rays, in addition to the above-listed characteristics.